Volcano

A volcano is a vent in the earth's crust through which magma, rock fragments, gases, and ash are ejected from the earth's interior. Over time, accumulation of these erupted products on the earth's surface creates a volcanic mountain.

Washington State has five major volcanoes in the Cascade Range – Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens and Mount Adams.

What's it all mean?

Volcanoes can lie dormant for centuries between eruptions, and the risk posed by volcanic activity is not always apparent. When Cascades volcanoes do erupt, high speed avalanches of hot ash and rock called pyroclastic flows, lava flows, and landslides can devastate areas 10 or more miles away, while huge mudflows of volcanic ash and debris called lahars can inundate valleys more than 50 miles downstream. Falling ash from explosive eruptions can disrupt human activities hundreds of miles downwind and drifting clouds of fine ash can cause severe damage to the engines of jet aircraft hundreds or thousands of miles away.

Washington’s volcanoes will erupt again, as shown by activity at Mount St. Helens. Mount St. Helens last erupted in 2004-2008, when it produced a series of spectacular lava spines with a cumulative volume of almost 100 million cubic meters, according to the Pacific Northwest Seismic Network. Tremor and millions of small earthquakes accompanied both of the recent eruptions.

Because people are moving into areas near these mountains at a rapid pace, the state’s volcanoes are among the most dangerous in the United States.

Mount Baker in Whatcom County erupted in the mid-1800s for the first time in several thousand years. Activity at steam vents in Sherman Crater, near the volcano's summit, increased in 1975 and is still vigorous, but there is no evidence that an eruption is imminent.

Glacier Peak in Snohomish County has erupted at least six times in the past 4,000 years. An especially powerful series of eruptions about 13,000 years ago deposited volcanic ash at least as far away as Wyoming.

Mount Rainier in Pierce County is one of the most hazardous volcanoes in the United States. It has produced at least four eruptions and numerous lahars in the past 4,000 years. It is capped by more glacier ice than the rest of the Cascades volcanoes combined, and Rainier's steep slopes are under constant attack from hot, acidic volcanic gases and water. These factors make this volcano especially prone to landslides and lahars. More than 150,000 people live on former lahars in river valleys below the volcano.

Mount St. Helens in Skamania County is the most frequently active volcano in the Cascades. During the past 4,000 years, it has produced many lahars and a wide variety of eruptive activity, from relatively quiet outflows of lava to explosive eruptions much larger than that of May 18, 1980. The recent eruption cycle lasted between 2004 and 2008 and built up the cone on the summit. In recent years, smoke can sometimes be seen.

Mount Adams in Yakima County has produced few eruptions during the past several thousand years. This volcano's most recent activity was a series of small eruptions about 1,000 years ago followed by a debris avalanche and lahar that inundated part of the Trout Lake lowland less than 500 years ago.

Additionally, Oregon’s Mount Hood, about 50 miles southeast of Portland, poses some threat to areas of Southwest Washington along the Columbia River. Mount Hood has erupted repeatedly for thousands of years, most recently during two episodes in the past 1,500 years; the last eruption ended shortly before the arrival of Lewis and Clark in 1805. Mount Hood, and other volcanoes in British Columbia, Oregon, and California, can produce tephra, which will fall on and impact Washington. The April 2005 USGS assessment states the threat posed by Mount Hood also is very high.